linux-kernel - Re: [PATCH 2/9] IB: add a proper completion queue abstraction

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Date:	Wed, 18 Nov 2015 10:20:14 -0800
From:	Bart Van Assche <bart.vanassche@...disk.com>
To:	Sagi Grimberg <sagig@....mellanox.co.il>,
	Christoph Hellwig <hch@....de>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>
CC:	"axboe@...com" <axboe@...com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/9] IB: add a proper completion queue abstraction

On 11/17/2015 11:55 PM, Sagi Grimberg wrote:
>>> +static void ib_cq_poll_work(struct work_struct *work)
>>> +{
>>> +    struct ib_cq *cq = container_of(work, struct ib_cq, work);
>>> +    int completed;
>>> +
>>> +    completed = __ib_process_cq(cq, IB_POLL_BUDGET_WORKQUEUE);
>>> +    if (completed >= IB_POLL_BUDGET_WORKQUEUE ||
>>> +        ib_req_notify_cq(cq, IB_POLL_FLAGS) > 0)
>>> +        queue_work(ib_comp_wq, &cq->work);
>>> +}
>>> +
>>> +static void ib_cq_completion_workqueue(struct ib_cq *cq, void *private)
>>> +{
>>> +    queue_work(ib_comp_wq, &cq->work);
>>> +}
>>
>> The above code will cause all polling to occur on the context of the CPU
>> that received the completion interrupt. This approach is not powerful
>> enough. For certain workloads throughput is higher if work completions
>> are processed by another CPU core on the same CPU socket. Has it been
>> considered to make the CPU core on which work completions are processed
>> configurable ?
>
> The workqueue is unbound. This means that the functionality you are
> you are asking for exists.

Hello Sagi,

Are you perhaps referring to the sysfs CPU mask that allows to control 
workqueue affinity ? I expect that setting the CPU mask for an entire 
pool through sysfs will lead to suboptimal results. What I have learned 
by tuning target systems is that there is a significant performance 
difference (> 30% IOPS) between a configuration where each completion 
thread is pinned to exactly one CPU compared to allowing the scheduler 
to choose a CPU.

Controlling the CPU affinity of worker threads with the taskset command 
is not possible since the function create_worker() in kernel/workqueue.c 
calls kthread_bind_mask(). That function sets PF_NO_SETAFFINITY. From 
sched.h:

#define PF_NO_SETAFFINITY 0x04000000 /* Userland is not allowed to 
meddle with cpus_allowed */

Bart.
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